You can lock in any legendary that you like.
Firstly, make sure to press the “unlock all” link next to the Start Over button at the top of the gear table. If you don’t see an unlock all link, then everything is already unlocked.
Then, e.g. if you want to force it to use a legendary helmet, first run BiB, then click on the item it chose in the head slot. A list pops up with all of the helmets that you own. Choose the legendary. It will now have a dotted border around it in the gear table.
Do that for your other legendary as well.
Then press Save and Update at the top of the gear table.